View metadata, citation and similar papers at core.ac.uk brought to you by CORE provided by Frontiers - Publisher Connector REVIEW published: 24 September 2015 doi: 10.3389/fpls.2015.00684 Agave as a model CAM crop system for a warming and drying world J. Ryan Stewart * Department of Plant and Wildlife Sciences, Brigham Young University, Provo, UT, USA As climate change leads to drier and warmer conditions in semi-arid regions, growing resource-intensive C3 and C4 crops will become more challenging. Such crops will be subjected to increased frequency and intensity of drought and heat stress. However, agaves, even more than pineapple (Ananas comosus) and prickly pear (Opuntia ficus-indica and related species), typify highly productive plants that will respond favorably to global warming, both in natural and cultivated settings. With nearly 200 species spread throughout the U.S., Mexico, and Central America, agaves have evolved traits, including crassulacean acid metabolism (CAM), that allow them to survive extreme heat and drought. Agaves have been used as sources of food, beverage, and fiber by societies for hundreds of years. The varied uses of Agave, combined with its unique adaptations to environmental stress, warrant its consideration as a model CAM crop. Besides the damaging cycles of surplus and shortage that have long beset the tequila industry, the Edited by: relatively long maturation cycle of Agave, its monocarpic flowering habit, and unique Edmundo Acevedo, University of California, Davis, USA morphology comprise the biggest barriers to its widespread use as a crop suitable for Reviewed by: mechanized production. Despite these challenges, agaves exhibit potential as crops Nicolas Franck, since they can be grown on marginal lands, but with more resource input than is widely Universidad de Chile, Chile assumed. If these constraints can be reconciled, Agave shows considerable promise as Victor Garcia De Cortazar, Universidad de Chile, Chile an alternative source for food, alternative sweeteners, and even bioenergy. And despite *Correspondence: the many unknowns regarding agaves, they provide a means to resolve disparities in J. Ryan Stewart, resource availability and needs between natural and human systems in semi-arid regions. Department of Plant and Wildlife Sciences, Brigham Young University, Keywords: Agave, agriculture, bioenergy, CAM, century plant, stress physiology, succulent 4105 Life Sciences Building, Provo, UT 84602, USA [email protected] Introduction Specialty section: As arable land continues to degrade and diminish, and as the Earth’s population continues to spiral This article was submitted to upward (Gerland et al., 2014) in a warming and drying world (Fischer et al., 2005; Howden et al., Crop Science and Horticulture, 2007; Dai, 2013; Cook et al., 2014), demand will ramp up for high-yielding crops that will be a section of the journal Frontiers in Plant Science productive in semi-arid and arid regions. Few C3 or C4 crop species, however, exhibit traits that enable them to be sustainably productive in dry and hot climates, particularly in nutrient-poor soils Received: 12 May 2015 (Pimienta-Barrios, 1994; Lobell and Field, 2007; Somerville et al., 2010; Davis et al., 2011; Challinor Accepted: 17 August 2015 Published: 24 September 2015 et al., 2014; Owen and Griffiths, 2014). Photosynthesis via the C4 pathway enables several crops, such as corn (Zea mays), sugarcane (Saccharum officinarum), and sorghum (Sorghum bicolor), Citation: Stewart JR (2015) Agave as a model to tolerate extremes in heat, but maintaining high yields depends on having reliable sources of CAM crop system for a warming and irrigation water (Jaggard et al., 2010; Vanloocke et al., 2010; Le et al., 2011; Black et al., 2012; drying world. Front. Plant Sci. 6:684. Zhuang et al., 2013; Goldstein et al., 2014; Yimam et al., 2014). The need for heat- and drought- doi: 10.3389/fpls.2015.00684 tolerant crops is especially pronounced in arid and semi-arid regions, which cover nearly 40% of Frontiers in Plant Science | www.frontiersin.org 1 September 2015 | Volume 6 | Article 684 Stewart Agave as a model CAM crop the world’s land surface area (Ramankutty et al., 2008; low stomatal conductance, largely allow these plants to survive Vorosmarty et al., 2010; Borland et al., 2014). In these regions, in semi-arid and arid lands (Szarek et al., 1973; Cushman, insufficient rainfall and intense evapotranspiration act as barriers 2001; Matiz et al., 2013). Several CAM crops exist, such as to the cultivation of many economically important C3 and C4 Aloe vera (Xanthorrhoeaceae), Hylocereus spp. (Cactaceae), and crops (Borland et al., 2009). Vanilla planifolia (Orchidaceae) (Yang et al., 2015), but several Although limited in number, some crop species fix CO2 Agave species (Asparagaceae); pineapple (Ananas comosus, through an alternate form of photosynthesis, crassulacean acid Bromeliaceae); and prickly pear cactus (Opuntia ficus-indica, metabolism (CAM), which maximizes water-use efficiency by Cactaceae) and other species in the genus, have been cultivated shifting most CO2 uptake to the night (Borland et al., 2009; Yang for hundreds of years for food, beverages, and fiber in hot and et al., 2015). Cooler nighttime temperatures reduce the vapor drought-prone regions of the world (Gentry, 1982; Griffith, 2004; pressure gradient between their leaves and the air, resulting in Clement et al., 2010). The benefits and uses of these crops are markedly lower transpiration rates as compared to C3 and C4 outlined in Table 1. plants (Griffiths, 1988; Winter and Smith, 1996). Consequently, Given the dearth of efforts to develop and improve CAM CAM confers the ability to plants to be highly water-use species as crops, priority needs to be given to research on efficient in hot, water-limited environments. While CAM is species that will yield the greatest impact in addressing human found in several families, approximately 7% of all plant species needs. On a global scale, Ananas comosus ranks as the most use this photosynthetic pathway (Silvera et al., 2010). Most commercially important CAM crop (Py et al., 1987; Carr, 2012; CAM plants are small and lack any apparent benefits as crops; Yang et al., 2015). Besides being mainly grown for its fresh CAM and allied traits, such as succulence, waxy cuticles, and fruit in different parts of the world (Table 1), other uses of TABLE 1 | Key crassulacean acid metabolism (CAM) crops and information related to their historic, current, and potential end uses and their area of cultivation. Species Harvested part Historical and/or current uses Potential end uses Area of production Agave angustifolia Leaves, stem, sap Distilled alcoholic beverage (mezcal)a Bioenergy from soluble carbohydrates Mexico (Oaxaca)a and lignocelluloseb,c,d, medicinee, prebiotice, food additivef Agave mapisaga Leaves, stem, sap Non-distilled alcoholic beverage Bioenergy from soluble Mexico (Mexico, Tlaxcala, Hidalgo, (pulque)a,g carbohydrates and lignocellulosec Queretaro, Mexico DF, Puebla, Morelos, San Luis Potosi)a Agave salmiana Leaves, stem, sap Non-distilled, distilled alcoholic Bioenergy from soluble carbohydrates Mexico (San Luis Potosi)a beverage (pulque, mezcal)a,g, and lignocelluloseb,j, medicinee sweetener (syrup)h,i Agave tequilana Leaves, stem, sap Distilled alcoholic beverage Bioenergy from soluble carbohydrates Mexico (Jalisco, Guanajuato, (tequila)a,k, sweetener (syrup)h,i,l and lignocelluloseb,c,d,j,m, prebiotice Michoacan, Nayarit, Tamaulipas)n Agave fourcroydes Leaves Fibero,p, distilled alcoholic beverageq Bioenergy from soluble Mexico (Yucatan)v carbohydrates and lignocelluloser,s, leaf fiber as reinforcement agent for composite fiberst,u Agave lechuguilla Leaves Fiberw,x Leaf fiber as reinforcement agent for Mexico (Coahuila, Chihuahua, Nuevo composite fibersy, medicinez,aa,bb Leon, Durango, San Luis Potosi, and Zacatecas)cc Agave sisalana Leaves Fiberdd,ee Bioenergy from soluble carbohydrates Brazil, Kenya, Tanzanias,hh and lignocelluloses, leaf fiber as reinforcement agent for composite fibersff, medicinegg, bioinsecticidegg Ananas comosus Fruit, leaves Fruit, beverageii Bioenergy from soluble Brazil, Costa Rica, Phillipines, carbohydratesjj, leaf fiber as Thailandii reinforcement agent for composite fiberskk, medicinell, food additivemm Opuntia ficus-indica Fruit, cladodes Fruitnn, nopalesoo, foragepp, Bioenergy from soluble Algeria, Brazil, Chile, Mexico, Sicilyyy cochineal dyeqq carbohydrates and lignocelluloserr, antioxidantsss,tt,aa, medicineuu,vv,ww, dyesxx aLappe-Oliveras et al., 2008; bEscamilla-Trevino, 2012; cGarcia-Moya et al., 2011; dNúñez et al., 2011; eSantos-Zea et al., 2012; f Rivera et al., 2010; gOrtiz-Basurto et al., 2008; hWillems and Low, 2012; iGarcia-Pedraza et al., 2009; jDavis et al., 2011; k Cedeño Cruz, 2003; lMontanez Soto et al., 2011; mOwen and Griffiths, 2014; nZizumbo-Villarreal and Colunga-GarcíaMarín, 2008; oColunga-GarcíaMarín, 2003; pNobel, 1985; qRendon-Salcido et al., 2009; r Martinez-Torres et al., 2011; sDavis and Long, 2015; tGonzalez-Murillo and Ansell, 2009; uMay-Pat et al., 2013; vGarcía de Fuentes and Morales, 2000; wBautista Lopez and Martinez Cruz, 2012; xPando-Moreno et al., 2008; yVelasquez-Martinez et al., 2011; zMendez et al., 2012; aaSantos-Zea et al., 2012; bbRamos Casillas et al., 2012; ccPando-Moreno et al., 2004; ddColunga-GarciaMarin and May-Pat, 1993; eeLock, 1962; ff Barreto et al., 2011; ggRibeiro et al., 2013; hhKimaro et al., 1994; iiCarr, 2012; jjWang et al., 2006; kk Mohanty et al., 2000; llGupta et